Breast cancer is by far the most common cancer among women. Each year, more than 180,000 and 1 million women in the U.S. and worldwide, respectively, are diagnosed with breast cancer. Breast cancer is the leading cause of death for women between ages 50-55, and is the most common non-preventable malignancy in women in the Western Hemisphere. An estimated 2,167,000 women in the United States are currently living with the disease (National Cancer Institute, Surveillance Epidemiology and End Results (NCI SEER) program, Cancer Statistics Review (CSR), www-seer.ims.nci.nih.gov/Publications/CSR1973 (1998)). Based on cancer rates from 1995 through 1997, a report from the National Cancer Institute (NCI) estimates that about 1 in 8 women in the United States (approximately 12.8 percent) will develop breast cancer during her lifetime (NCI's Surveillance, Epidemiology, and End Results Program (SEER) publication SEER Cancer Statistic's Review 1973-1997). Breast cancer is the second most common form of cancer, after skin cancer, among women in the United States. An estimated 250,100 new cases of breast cancer are expected to be diagnosed in the United States in 2001. Of these, 192,200 new cases of more advanced (invasive) breast cancer are expected to occur among women (an increase of 5% over last year), 46,400 new cases of early stage (in situ) breast cancer are expected to occur among women (up 9% from last year), and about 1,500 new cases of breast cancer are expected to be diagnosed in men (Cancer Facts & FIGS. 2001 American Cancer Society). An estimated 40,600 deaths (40,300 women, 400 men) from breast cancer are expected in 2001. Breast cancer ranks second only to lung cancer among causes of cancer deaths in women. Nearly 86% of women who are diagnosed with breast cancer are likely to still be alive five years later, though 24% of them will die of breast cancer after 10 years, and nearly half (47%) will die of breast cancer after 20 years.
Every woman is at risk for breast cancer. Over 70 percent (Abreast cancers occur in women who have no identifiable risk factors other than age (U.S. General Accounting Office. Breast Cancer, 1971-1991: Prevention, Treatment and Research. GAO/PEMD-92-12; 1991). Only 5 to 10% of breast cancers are linked to a family history of breast cancer (Henderson I C, Breast Cancer. In: Murphy G P, Lawrence W L, Lenhard R E (eds). Clinical Oncology. Atlanta, Ga.: American Cancer Society; 1995:198-219).
Each breast has 15 to 20 sections called lobes. Within each lobe are many smaller lobules. Lobules end in dozens of tiny bulbs that can produce milk. The lobes, lobules, and bulbs are all linked by thin tubes called ducts. These ducts lead to the nipple in the center of a dark area of skin called the areola. Fat surrounds the lobules and ducts. There are no muscles in the breast, but muscles lie under each breast and cover the ribs. Each breast also contains blood vessels and lymph vessels. The lymph vessels carry colorless fluid called lymph, and lead to the lymph nodes. Clusters of lymph nodes are found near the breast in the axilla (under the arm), above the collarbone, and in the chest.
Breast tumors can be either benign or malignant. Benign tumors are not cancerous, they do not spread to other parts of the body, and are not a threat to life. They can usually be removed, and in most cases, do not come back. Malignant tumors are cancerous, and can invade and damage nearby tissues and organs. Malignant tumor cells may metastasize, entering the bloodstream or lymphatic system. When breast cancer cells metastasize outside the breast, they are often found in the lymph nodes under the arm (axillary lymph nodes). If the cancer has reached these nodes, it means that cancer cells may have spread to other lymph nodes or other organs, such as bones, liver, or lungs.
Major and intensive research has been focussed on early detection, treatment and prevention. This has included an emphasis on determining the presence of precancerous or cancerous ductal epithelial cells. These cells are analyzed, for example, for cell morphology, for protein markers, for nucleic acid markers, for chromosomal abnormalities, for biochemical markers, and for other characteristic changes that would signal the presence of cancerous or precancerous cells. This has led to various molecular alterations that have been reported in breast cancer, few of which have been well characterized in human clinical breast specimens. Molecular alterations include presence/absence of estrogen and progesterone steroid receptors, HER-2 expression/amplification (Mark H F, et al. HER-2/neu gene amplification in stages I-IV breast cancer detected by fluorescent in situ hybridization. Genet Med; 1(3):98-103 1999), Ki-67 (an antigen that is present in all stages of the cell cycle except G0 and used as a marker for tumor cell proliferation, and prognostic markers (including oncogenes, tumor suppressor genes, and angiogenesis markers) like p53, p27, Cathepsin D, pS2, multi-drug resistance (MDR) gene, and CD31.
Examination of cells by a trained pathologist has also been used to establish whether ductal epithelial cells are normal (i.e. not precancerous or cancerous or, having another noncancerous abnormality), precancerous (i.e. comprising hyperplasia, atypical ductal hyperplasia (ADH)) or cancerous (comprising ductal carcinoma in situ, or DCIS, which includes low grade ductal carcinoma in situ, or LG-DCIS, and high grade ductal carcinoma in situ, or HG-DCIS) or invasive (ductal) carcinoma (IDC). Pathologists may also identify the occurrence of lobular carcinoma in situ (LCIS) or invasive lobular carcinoma (ILC). Breast cancer progression may be viewed as the occurrence of abnormal cells, such as those of ADH, DCIS, IDC, LCIS, and/or ILC, among normal cells.
It remains unclear whether normal cells become hyperplastic (such as ADH) and then progressing on to become malignant (DCIS, IDC, LCIS, and/or ILC) or whether normal cells are able to directly become malignant without transitioning through a hyperplastic stage. It has been observed, however, that the presence of ADH indicates a higher likelihood of developing a malignancy. This has resulting in treatment of patients with ADH to begin treatment with an antineoplastic/antitumor agent such as tamoxifen. This is in contrast to the treatment of patients with malignant breast cancer which usually includes surgical removal.
The rational development of preventive, diagnostic and therapeutic strategies for women at risk for breast cancer would be aided by a molecular map of the tumorigenesis process. Relatively little is known of the molecular events that mediate the transition of normal breast cells to the various stages of breast cancer progression. Similarly, little is known of the molecular events that mediate the transition of cells from one stage of breast cancer to another.
Molecular means of identifying the differences between normal, non-cancerous cells and cancerous cells (in general) have been the focus of intense study. The use of cDNA libraries to analyze differences in gene expression patterns in normal versus tumorigenic cells has been described (U.S. Pat. No. 4,981,783). DeRisi et al. (1996) describe the analysis of gene expression patterns between two cell lines: UACC-903, which is a tumorigenic human melanoma cell line, and UACC-903(+6), which is a chromosome 6 suppressed non-tumorigenic form of UACC-903. Labeled cDNA probes made from mRNA from these cell lines were applied to DNA microarrays containing 870 different cDNAs and controls. Genes that were preferentially expressed in one of the two cell lines were identified.
Golub et al. (1999) describe the use of gene expression monitoring as means to cancer class discovery and class prediction between acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). Their approach to class predictors used a neighborhood analysis followed by cross-validation of the validity of the predictors by withholding one sample and building a predictor based only on the remaining samples. This predictor is then used to predict the class of the withheld sample. They also used cluster analysis to identify new classes (or subtypes) within the AML and ALL.
Gene expression patterns in human breast cancers have been described by Perou et al. (1999), who studied gene expression between cultured human mammary epithelia cells (HMEC) and breast tissue samples by use of microarrays comprising about 5000 genes. They used a clustering algorithm to identify patterns of expression in HMEC and tissue samples. Peron et al. (2000) describe the use of clustered gene expression profiles to classify subtypes of human breast tumors. Hedenfalk et al. describe gene expression profiles in BRCA1 mutation positive, BRCA2 mutation positive, and sporadic tumors. Using gene expression patterns to distinguish breast tumor subclasses and predict clinical implications is described by Sortie et al. and West et al.
All of the above described approaches, however, utilize heterogeneous populations of cells found in culture or in a biopsy to obtain information on gene expression patterns. The use of such populations may result in the inclusion or exclusion of multiple genes from the patterns. For this and the lack of statistical robustness reasons, the gene expression patterns observed by the above described approaches provide little confidence that the differences in gene expression may be meaningfully associated with the stages of breast cancer.
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